Traditionally, medical diagnostic processes record x-ray image patterns on silver halide films. These systems direct an initially uniform pattern of interrogating x-ray radiation through a patient to be studied, intercept the consequently imagewise modulated pattern of x-ray radiation with an x-ray radiation intensifying screen, record the intensified pattern in a silver halide film, and chemically transform the latent radiation pattern into a permanent and visible image called a radiogram.
Radiograms have also been produced by using layers of radiation sensitive materials to directly capture radiographic images as imagewise modulated patterns of electrical charges. Depending upon the intensity of the incident X-ray radiation, electrical charges generated either electrically or optically by the X-ray radiation within a pixelized area are quantized using a regularly arranged array of discrete solid state radiation sensors.
There has been rapid development of large area, flat panel, digital x-ray imaging detectors for digital radiology using active matrix technologies. An active matrix consists of a two-dimensional array of thin film transistors (TFTs) made with amorphous or polycrystalline semiconductor materials. There are two general approaches to making flat-panel x-ray detectors, direct or indirect. The direct method is also referred to as a self-scanned .alpha.-Se (amorphous selenium). The indirect method uses phosphor screens or other scintillators to first convert x-rays to visible light, which is then read out with an active matrix array with an additional light sensor at each pixel of the array. The direct method offers the advantages of higher screen resolution provided by electrostatic image formation, elimination of the phosphor screen, and simpler active matrix structure.
The usefulness of photoconductors as x-ray imaging sensors arises from the high sensitivity, i.e., conversion of absorbed x-ray energy to charge, if an appropriate electric field is applied. While a number of photoconductors may be used for x-ray imaging detectors, amorphous selenium is considered particularly well-suited for the task, since it is well-developed technologically, it has been used traditionally as a photoconductor in photocopiers and xeroradiography, it can be made relatively easily and inexpensively in its amorphous form by evaporation in a large area, and it has a uniqueness in its remarkably low dark current. Further, a large intrinsic gain results from its large difference between the hole mobility and electron mobility for ohmic contacts.
While achieving advantages over traditional film radiography, photoconductor use in x-ray imaging has its share of difficulties. Image lag is a problem when utilizing direct x-ray detection. Image lag or a delay in acquiring an image is effected by two sources. A first source is incomplete charge collection i.e., photoconductive lag and a second source is incomplete charge readout i.e., image readout lag. What is desired is a system and method for reducing the photoconductive lag in a cost effective and efficient manner. The present invention addresses such a need.